Known compressor elements for the said type comprise a housing with an inlet for the gas on the inlet side and an outlet for the gas on the outlet side and two rotor chambers in which two helical rotors are mounted on bearings that mesh together when driven to compress the gas, respectively a male rotor with a drive gearwheel for driving the male rotor by a gearwheel transmission and a female rotor that is driven by the male rotor by means of synchronisation gearwheels with at least one synchronisation gearwheel on the male rotor and one synchronisation gearwheel on the female rotor, whereby the synchronisation gearwheels are generally designed such that, when driven, the male rotor rotates faster than the female rotor.
By driving the compressor element, chambers are formed between the two rotors that are filled with gas at the inlet, whereby upon the rotation of the rotors these chambers move from the inlet side to the outlet side and become increasingly smaller, such that the enclosed gas is compressed and which is delivered at a higher pressure to a downstream consumer network via a pressure pipe connected to the outlet.
Due to the compression, forces are exerted by the gases on the rotors that tend to push the rotors away from the outlet side in the direction of the inlet side.
The drive gearwheel on the male rotor is chosen such that when driven by the drive gearwheel a force is exerted with an axial component that is directed from the inlet to the outlet, thus oriented opposite to the axial component of the force exerted by the gas on the male rotor, so that this gas force is partially offset by the driving force of the drive gearwheel, so that the axial bearings are exposed to smaller forces.
The synchronisation gearwheels also exert a force on the rotors, whereby this force on the male rotor generally adds to the gas force on this rotor, while in the case of the female rotor this force counteracts the gas force.
When the compressor element is driven unloaded, in other words without compressed gas having to be supplied, the gas forces are non-existent or minimal, such that the composite forces of the drive gearwheel and the synchronisation gearwheels could tend to push the rotors in the opposite direction towards the outlet, in contrast to the loaded situation with the supply of compressed gas.
During dynamic transitional modes, forces can occur that push the rotors in the one or the other direction.
All this means that the direction of the composite forces that are exerted on the rotors depends on the mode, loaded or unloaded, and the fact that the situation is static or dynamic, such that in some circumstances these forces tend to push the rotors against the inlet end face of the housing on the inlet side, and in other circumstances against the outlet end face of the housing on the outlet side.
To prevent the rotors coming into contact with one of the two end faces of the housing, the rotors are generally axially fixed by two axial bearings, more specifically one on the inlet side and one on the outlet side, complemented with a radial bearing on either side of the rotors.
It is known to provide compressor elements of the screw type with means in the form of a spring or a plunger to exert an additional mechanical axial force or prestress force on each rotor, in order to relieve the bearings and/or to prevent, in the absence of gas forces in the unloaded mode, the rotors being pushed or pulled by the axial drive forces of the drive gearwheel and the synchronisation gearwheels against the housing. These means are generally built into the bearing cover, such that it must be made extra thick and heavy.
A disadvantage of such force means is that it detrimentally affects the costs of the compressor element and that in some circumstances it also increases the load on the bearings instead of offsetting them such that larger bearings are required.
If plungers are used as a force means, the exerted force can be controlled, but such a control entails extra costs, makes the compressor element more vulnerable to possible breakdowns and increases the size and mass of the bearing cover and therefore also the forces and vibrations on the housing of the compressor element.
The axle journal of the male rotor on which the drive gearwheel is mounted experiences a relatively high bending force due to the radial forces that are exerted by the drive gearwheel on it. This has the disadvantage that in certain extreme conditions the axial bearing of the male rotor that is mounted on this axle journal can tilt, which can lead to a limitation of the operating region of the compressor element.
The known compressor elements of the discussed type are outlet driven, which means that the gearwheel transmission with the drive gearwheel is on the hot outlet side of the compressor element, whereby the axial bearing on this side of the rotors is in contact with the less pure environment of the gearwheel transmission, which can affect their lifetime. This axial bearing is called the main bearing and its primary function is to retain the rotor concerned locally in the axial direction.
Due to a varying temperature gradient in the axial direction of the rotors as a function of the mode of the compressor element, changes also occur in the length of the shaft of the rotors, whereby different temperatures of the male and the female rotors lead to different length changes of the two rotors and thus a change in the mutual axial position of both synchronisation gearwheels. This mutual axial displacement of the synchronisation gearwheels, in the event of synchronisation gearwheels with oblique toothing, has the undesired effect that the synchronisation between the rotors changes with the temperature.
With the known outlet-driven compressor elements the synchronisation gearwheels are on the inlet side in other words on the opposite side of the rotor where the main bearing is located and thus at a relatively large distance from this main bearing, such that the synchronisation gearwheels experience a significant mutual axial displacement due to the differential length variations of the rotors as a result of varying temperature gradients, with the disadvantage that in the case of synchronisation gearwheels with oblique toothing, the synchronisation change between the male and the female rotor can be undesirably large.